With respect to related technical ideas, as moving picture compressing/encoding systems capable of transmitting moving picture data in low bit rates and in higher efficiencies when the moving picture data are transmitted, the moving picture compressing/encoding systems called “H.261” and “H.263” have been proposed based upon recommendation made by ITU-T (International Telecommunications Union-Telecommunication Standardization Sector).
Also, as other moving picture compressing/encoding systems, a system called MPEG-4 (Moving Picture Experts Group) has been proposed which has been internationally standardized by ISO/IEC (International Organization for Standardization/International Electrotechinal Commission).
Further, very recently, another moving picture compressing/encoding system called H.264/MPEG-4 AVC (Advanced Video Coding) which has been internationally standardized by ITU-T and ISO/IEC is attracting attention as the system capable of encoding and of transmitting moving picture data at higher efficiency than the moving picture compressing/encoding systems such as “H.261”, “H.263”, and “MPEG-4.”
Among the above-mentioned moving picture compressing/encoding systems, there are proposed: an intra-prediction by which encoding is performed by employing only image data of a present frame; and an inter-prediction by which encoding is performed by referring to image data of a preceding (past) frame and image data of a succeeding (future) frame (refer to, for example, Patent Documents 1 and 2).
In the intra-prediction, as one example, when moving picture data is encoded, the moving picture data is segmented in a unit called a macroblock (for example, a block of 16×16 pixels). Then, in this intra-prediction, the moving picture data is encoded by the DCT (Discrete Cosine Transform) for each macroblock, and thereafter, a DCT coefficient obtained by the DCT transform is variable-length-coded as the encoding process for encoding the moving picture data for each macroblock at higher efficiency.
The above-mentioned DCT implies such a transforming technique that image data of a frame is resolved into frequency components such as a high frequency component and a low frequency component, and then, an encoding processing is carried out with respect to those frequency components. Also, in the H.264/MPEG-4 AVC system, the following method has been employed: while a block (4×4 pixels) is employed by further subdividing a macroblock of 8×8 pixels, a DIT (Discrete Integer Transform) is performed for the block, and thereafter, a DIT coefficient obtained by this DIT transform is variable-length-coded. The DIT transform can encode moving picture data at higher efficiency than that of the above-mentioned DCT transform.
In this case, a variable length coding operation implies the following encoding operation: when moving picture data is encoded, data with a small number of bytes is allocated to a higher appearance frequency value (DCT coefficient, DIT coefficient), whereas data with a large number of bytes is allocated to a lower appearance frequency value. As a consequence, the encoding efficiency can be further increased.
On the other hand, in the inter-prediction, a prediction image is produced based upon frames at different time instants, and data of a difference image between an input image and the prediction image is encoded. In this case, in accordance with the inter-prediction, since only the data of the difference image needs to be encoded, a higher encoding rate may be achieved, as compared with that of the intra-prediction.
Also, in this inter-prediction, in order to achieve a higher encoding rate, a motion compensating technique has been utilized. This motion compensation implies such an image encoding system that an encoding operation or the like is carried out by considering to which direction an object of interest has moved between two continuous frames. In this motion compensation, first of all, a motion vector (information for representing how long each of elements contained in a picture has been moved to which direction) is acquired between the adjoining frames. After this motion component has been removed, image data is encoded by employing the DCT, or the like, and then, encoded data which has encoded the image data is produced. In this case, when this encoded data is decoded, the encoded data is decoded based upon the motion vector information.
Also, generally speaking, three picture types are provided in the above-mentioned moving picture compressing/encoding systems, namely, an I picture (Intra-Picture), a P picture (Predictive Picture), and also, a B picture (Bidirectional Predictive Picture).
The I picture implies such a picture which is obtained by independently encoding a relevant frame irrespective of frames located before and after the frame. The P picture implies such a picture which is obtained by predictive-coding frames along a forward direction. The B picture implies such a picture which is obtained by predictive-coding frames along bidirectional direction, namely, the forward direction and the backward direction.
In this case, the above-mentioned Patent Document 1 discloses the technique related to the motion compensation among the moving picture compressing/encoding systems. Specifically, in the inter-prediction, in such a case where not only a forward direction prediction, but also a backward direction prediction is performed for a frame to be predicted, a motion vector is acquired by employing frames of bidirectional directions, and then, the motion-compensated prediction is carried out.
Also, the above-mentioned Patent Document 2 discloses the technique related to the motion compensation among the moving picture compressing/encoding systems. Specifically, in the inter-prediction, not only a forward direction prediction, but also a backward direction prediction is performed for a frame to be predicted so as to acquire the motion vector by employing the predicted frame having the smaller error, and then, the motion-compensated prediction is carried out.
On the other hand, in a moving picture transcoding apparatus which employs the above-mentioned moving picture compressing/encoding system, for example, in such a case where data encoded by the H.261 system is once stored in a memory, or the like, and then used, a first frame of stored image data (encoded data) cannot refer to image data of a preceding frame when the encoded data is reproduced (decoded). As a consequence, the first frame must be necessarily encoded by utilizing only the intra-prediction. This reason is that the decoded image is disturbed.
In this case, when it is desired to store frames of encoded data from a designated frame, if a P picture is the first frame, and then the P picture is directly decoded, there is a problem that the P picture cannot be correctly reproduced.
As a consequence, when the frames are stored from the designated frame, this designated frame is once decoded. Then, this decoded image is again encoded as the first frame by utilizing the intra-prediction. Also, a frame subsequent to this first frame is once decoded, and thereafter, is again encoded by employing the inter-prediction. However, since the frames are again encoded, there is such a problem that the image quality of the image is deteriorated. Also, there is another problem that processing loads are increased when the frames are again encoded. Accordingly, other technical ideas capable of solving the above-mentioned problems have been proposed (refer to, for example, Patent Document 3).
In the technique disclosed in Patent Document 3, in such a case where encoded data is stored from encoded data of a designated frame and then used, the encoded data (P picture) of the designated frame is once decoded. Subsequently, when the decoded data (P picture) is again encoded, either a first frame of a decoded image of the designated frame or each of several frames subsequent to this first frame is transcoded into an I picture by the intra-prediction. Then, the encoded data subsequent to the first frame is directly stored. As a result, since the first frame is transcoded into the I picture, the decoded image (picture) is prevented from being disturbed.    Patent Document 1: JP-A-2004-165703    Patent Document 2: JP-A-2006-180173    Patent Document 3: JP-A-2002-305733